Heat-shrinkable polyester-based films have been widely used for packaging and labeling of beverage containers (e.g., PET bottles) and other types of containers due to their heat-shrinkable characteristics. Conventionally, in order to label an ordinary beverage container with a cylindrical shape, a polyester-based film is cut into certain sizes and rolled into a cylindrical form. After both ends of the film are glued with an adhesive solvent, the cylindrical film is put around the container and then shrunk. For performing effective labeling, not only the characteristics of the heat-shrinkable polyester-based film are of great importance but also the characteristics of the adhesive solvent.
Conventionally, such solvents as chloroform, 1-methyl-2-pyrrolidinone, tetrahydrofuran (THF), and 1,3-dioxolane have been widely used as adhesive solvents for heat-shrinkable polyester-based labels. However, the use of chloroform is restricted because of its carcinogenicity; and 1-methyl-2-pyrrolidinone is not preferred due to its strong and unpleasant odor. Hence, THF and 1,3-dioxolane have been used singly or as a mixture thereof. However, as THF and 1,3-dioxolane are prone to oxidation to form peroxides, a long-term storage after exposure to air may lead to a decrease in their solubility, which would eventually bring about defects in the adhesion of the labels. As a result, heat-shrinkable film manufacturers have always had to use fresh solvents to secure consistent adhesive strength, suffering difficulties in the storage of solvents.
Therefore, it is necessary to develop a novel solvent for heat-shrinkable polyester-based labels, which has a long shelf life with consistent adhesive strength and excellent storage stability.